Plasticity is the property of a substance to deform without rupturing or the resistance offered by a fluid to flow freely. Plastic viscosity (PV) is a parameter of the Bingham plastic model and gives a measure of the resistance offered by the drilling fluid to flow freely. This resistance is a result of friction between the liquid undergoing deformation under shear stress and the solids and liquids present in the drilling mud.
A viscometer is used to measure the PV and yield point of drilling fluids. PV is an important rheological characteristic that affects the properties of drilling fluid.
Trenchlesspedia Explains Plasticity
Low PV indicates that the viscosity of mud exiting the bit is low and thus capable of drilling rapidly. High PV indicates excess colloidal solids and a viscous base fluid that will slow down the drilling speed. To achieve lower PV, the mud can be diluted to lower solids content. An increase in solids content in drilling mud such as barite, presence of shale particles and loss of fluid will increase the PV.
PV is measured using a viscometer by measuring the shear rate at 600, 300, 200, 100, 6 and 3 rounds per minute (rpm). PV value is obtained by subtracting the 600 rpm reading from the 300 rpm reading. However; increasing the PV of the mud can help increase the amount of recovered cuttings.